A survey by the national youth organisation found that more than one in four men aged 19 to 29 had experienced sex before marriage. About 13% of such cases resulted in unwanted pregnancies that led to abortions. Sex outside marriage and abortion are outlawed under Iran's Islamic legal code.

The survey also revealed that the average marrying age had risen to 40 for men and 35 for women, a blow to the government's goal of promoting marriage to shore up society's Islamic foundations.

The statistics were disclosed by the national youth organisation's social-cultural deputy, Ali Alkbar Asarnia, at a conference celebrating family values and were widely reported in Iranian media. However, the organisation later attempted to dismiss the findings as based on an unrepresentative sample and attacked media outlets that reported them.

Asarnia said Iran had around 15 million single young people and that 1.5 million more were becoming eligible for marriage each year. Seven million were already past the government's recommended marrying guideline age of 29. The trend was producing the "unpleasant and dangerous social side effects" of premarital sex, Asarnia said.

The government has already tried to boost the marriage rate, which had an unprecedented 1.2% decline in 2005. President Mahmoud Ahmadinejad has introduced a £720m "Reza love fund" - named after one of Shia Islam's 12 imams - to provide marriage loans. Plans have been announced to establish marriage bureaux to help people find partners.

Many blame economic circumstances for their failure to marry, citing high inflation, unemployment and a housing shortage along with cultural traditions that expect brides' families to provide dowries and husbands to commit themselves to mehrieh, an agreed cash gift.

However, Hojatoleslam Ghasem Ebrahimipour, a sociologist, told Shabestan news agency that the trend was due to the availability of premarital sex, and feminism among educated women. "When a woman is educated and has an income, she does not want to accept masculine domination through marriage," he said.